Rats with a genetic predisposition to burn fat more slowly tend to put on weight more readily than rodents bred to resist becoming obese, a new study shows.

The findings could help explain why some people get fat more readily than others, Dr. Mark I. Friedman of the Monell Chemical Senses Center in Philadelphia, one of the study’s authors, told Reuters Health.

Just like the animals in the study, he explained, it may be harder for obesity-prone people to use energy from the fats they eat - as well as to burn fat from their own bodies - forcing them to overeat to get enough energy. Essentially, Friedman said, “they overeat because they’re getting fat.”

Previous studies in people have found that individuals who burn fat slowly are more likely to subsequently gain weight, Friedman and his colleague Dr. Hong Ji note in the current issue of the journal Metabolism: Clinical and Experimental. To determine if impaired fat oxidation might be involved in the development of obesity, the researchers fed a low-fat diet to rats bred to either be susceptible or resistant to obesity, and then switched them to a high-fat diet.

No weight difference developed between the two groups of rats while they were eating a low-fat diet, the researchers found. However, the obesity-prone rats showed a 35-percent lower rate of fatty acid oxidation. And after the animals were switched to a higher fat diet, the obesity-prone rats gained 36 percent more weight than the obesity-resistant rats, even though they consumed just 14 percent more calories.

The obesity-prone rats also showed reduced expression of genes involved in transporting fatty acids to the liver cells and burning them to make energy.

The findings imply that impairment of fatty acid oxidation may contribute to obesity in humans, Ji and Friedman conclude.

While fat metabolism may be at fault, Friedman added, people who want to control their weight should be paying attention to carbs, too. “As far as diet recommendations go, I personally would not pin it all on the fat. I would say that it really depends on a mix of fat and carbohydrates.”

Friedman and his colleagues are now looking into the enzymes involved in fatty acid oxidation and how their function might be altered in obesity-prone rats, which could provide an approach to treating or preventing obesity in humans.

SOURCE: Metabolism: Clinical and Experimental, August 2007.